In recent years, the use of MMCs (Modular Multilevel Converters) has been contemplated in self-commutated static var compensators such as STATCOM (Static Synchronous Compensator), direct-current transmission systems such as BTB (Back to Back) system, and motor drive inverters (for example, see WO2007/025828 (PTD 1)).
The MMC includes an arm configured with a plurality of unit converters connected in series. Each unit converter includes a switching element and a direct-current (DC) capacitor. The unit converter allows the switching element to perform switching operation to output voltage of the DC capacitor to the output terminal. In general, a semiconductor switching element capable of on/off control, such as IGBT (Insulated Gate Bipolar Transistor), is used as a switching element.